1. Field of the Invention
The present invention relates to a radio communication method, a radio communication system, a radio communication base station, a radio communication terminal station, and a radio communication program to be installed in equipment constituting said system, all of which are suitably applicable to the radio communication system for data communication. According to the present invention, they are particularly suitable for radio transmission based on OFDM modulation (Orthogonal Frequency Division Multiplexing).
2. Description of the Related Art
Attempts are being made to use for public multi-cell service the radio communication system which has originally been developed for the wireless LAN system to perform information transmission by means of OFDM modulation. OFDM modulation is a multi-carrier modulation scheme, in which one transmission channel available is divided into a plurality of subcarriers which are individually modulated with information to be transmitted. The radio communication system that employs OFDM modulation has good multi-path resistance and is suitable for mobile high-speed data communication because of its property that the duration of one OFDM symbol is longer than the delay time of multi-path delayed waves in mobile radio communication.
FIG. 29 is a diagram showing the arrangement of carries in the conventional radio communication system. This arrangement accords to the scheme called HiSWANa. According to this scheme, the frequencies of the center carrier are 5.17 GHz, 5.19 GHz, 5.21 GHz, and 5.23 GHz, and a signal band of 20 MHz (including a guard band) is assigned to each carrier.
FIG. 30 is a diagram showing the arrangement of subcarriers of one transmission channel in the conventional radio communication system. In each 20 MHz band shown in FIG. 29 are arranged the subcarriers, which have been generated by OFDM modulation, at intervals of 312.5 KHz. There are 53 subcarriers in total for information transmission. Of these 53 subcarriers, the subcarrier at the center (or the subcarrier centered at DC in the equivalent base band) is a null carrier which does not transmit information. (It corresponds to the subcarrier with a center frequency f0 in the carrier frequency band.) The frequency band used for information transmission is 16.5625 MHz, and both sides of the band are isolated from adjacent carriers by a guard band of about 1.7 MHz. This guard band is not used for information transmission.
FIG. 31 is a diagram showing an example of the format used to transmit control signals in the conventional radio communication system. In this system, the MAC frame is defined, which is a transmitting-receiving unit having a period of 2 ms. One radio frame has a length of 2 ms, and it is composed of four sections: broadcast burst, down-link phase, up-link phase, and contention phase. Incidentally, FIG. 31 shows only the broadcast burst and down-link phase, and the down-link phase is shown as down-link burst payload (DL burst payload).
The broadcast burst and down-link phase are sections for transmission from a base station to terminal stations. The broadcast burst is a section for transmission of control signals to all terminal stations under the control of the base station. The down-link phase is a section consisting mainly of a plurality of downbursts to transmit traffic data to each terminal station. The up-link phase and contention phase are sections for transmission from terminal stations to a base station. The broadcast burst includes broadcast preamble, BCH to transmit the base station information, FCH to transmit the allocation of traffic channel in the same frame to each terminal station, and ACH to reply to RCH used for calling from terminal stations.
The down-link phase includes SCH, which is a short traffic channel, and LCH, which is a long traffic channel. It is designed such that a plurality of SCH and/or LCH can jointly used for one mobile station in the period of down-link phase. This is called PDU (protocol data unit) train. The down-link preamble is attached to the head of each PDU train. One PDU train having a down-link preamble attached thereto is called down-link burst. In the period of up-link burst are contained SCH (which is a short traffic channel) and LCH (which is a long traffic channel). In the up-link, too, the PDU train is formed as in the case of down-link, and the up-link preamble is attached to the head of each PDU train. One PDU train having an up-link preamble attached thereto is called up-link burst. The contention phase contains RCH which is used for calling from mobile stations. To the head of each RCH is attached the up-link preamble to form the up-link burst.
The broadcast preamble has a length of 16 μs, and terminal stations receive this section to accomplish search of base stations, acquisition of initial synchronization, frame synchronization, frequency error correction, and symbol synchronization, after power has been turned on. The down-link preamble has a length of 8 μs, and terminal stations receive this section to accomplish more accurate timing correction, frequency error correction, and symbol synchronization. The up-link preamble has a length of 16 μs, and base stations receive this section to accomplish the timing correction, frequency error correction, and symbol synchronization for transmitting signals from terminal stations.
Such a system is constructed such that the terminal station calling signal is transmitted as traffic channel allocation information to each terminal station in FCH and the terminal station in the waiting mode waiting for being called judges whether or not it is called after it has received all of BCH and FCH in the broadcast burst.
Incidentally, there is another possible way of operation in which BCH and FCH in the broadcast burst in the head of all frames are not received but the frame intervals to be received by negotiation between a base station and a terminal station is thinned in order to increase the stand-by time in the terminal station.
FIG. 32 is a diagram showing the construction of a terminal station 300 in the ratio communication system that employs the conventional OFDM modulation. First, the construction of the transmission system will be explained step by step along the flow of signals. The data input/output processing unit 301 receives sound signals in the case of voice communication or data signals in the case of data communication by connection to a computer. It converts such signals into an adequate digital data string. The resulting output enters the transmitting data processing unit 311. If necessary, it receives from the control unit 302 communication control data to be transmitted to another OFDM radio equipment (base station), which is the called party of radio communication (not shown), and after multiplexing, it forms and outputs the frame and slot structure for transmission through the radio channel.
The output enters the CRC (Cyclic Redundancy Check) adding unit 312, for addition of redundancy to detect errors in the receiving end. The output from 312 enters the cipher unit 313. After encryption, the output from 313 enters the scrambler 314 in which scrambling is performed for pseudo randomization according to a prescribed algorithm. The output from 314 enters the encoding unit 315 in which error correction encoding is performed. There are several known methods for encoding, such as convolution coding, turbo coding, Reed-Solomon coding, and continuous coding (in which a plurality of coding methods are combined).
The output from the encoding unit 315 enters the interleaver 316, which performs interleaving (rearrangement of encoded bits according to a prescribed rule) so that the receiving end can convert burst errors into random errors by deinterleaving (reverse operation). The output from 316 enters the modulator 317, which, after mapping on the signal point at the time of transmission, outputs the in-phase component (I-component) and quadrature component (Q-component). The output from 317 enters the complex IFFT unit 318, which performs inverse fast Fourier transform for OFDM modulation.
The output from 318 enters the time waveform shaping unit 319, in which guard time is established by addition of cycleprefix and windowing is performed so as to smoothen the rise and fall of the OFDM modulation symbol. The output from 319 enters the DA converter 320, which performs conversion from digital waveform into analog waveform. The output from 320 enters the RF transmitter 321, which performs filtering, vector modulation for I-component and Q-component, frequency conversion into an adequate transmitting frequency channel, transmitting power control, and amplification.
The output from the RF transmitter 321 enters the antenna multiplexer 322. The output from 322 enters the antenna 323 to be eventually transmitted in the form of electromagnetic wave. The transmitted signals are received by another OFDM radio equipment (base station) which is the called party of radio communication (not shown). The antenna multiplex 323 is designed to separate transmitting signals and receiving signals from each other. Usually, an antenna switch is used for TDD system or FDD/TDMA system in which transmission and reception are accomplished in different timing, and a duplexer is used in other cases.
Next, the structure of the receiving system of the terminal station 300 will be explained. The signals which are received by the terminal station 300 are those which have been transmitted by the other OFDM radio equipment (base station) as the called party of radio communication (not shown). It is assumed that the transmitting signals have been formed by the same processing as in the terminal station 300 mentioned above.
The transmitting signal from the other OFDM radio equipment (base station) as the called party of radio communication (not shown) is received (in the form of electromagnetic wave) by the antenna 323. This signal is separated from the local transmitting signal by the antenna multiplexer 322, and the separated signal enters the RF receiver 331 which is the receiving circuit. The RF receiver 331 performs amplification, attenuation of undesired frequency components, selection of desired frequency channel, frequency conversion, level control of receiving signal amplitude, vector detection to separate I-component and Q-component from each other, and band limitation. It finally outputs I-component and Q-component. The output from the RF receiver 331 enters the AD converter 332, which performs conversion from analog waveform into digital waveform.
The output from 332 enters the synchronizing circuit 333, which performs frame synchronizing and frequency error correction. In the case where any party available for communication is searched immediately after power is turned on, the synchronizing circuit 333 performs synchronizing signal detection and initial synchronizing. The output from 333 enters the time waveform shaping unit 334, which performs time waveform shaping to remove guard time by addition of cycleprefix. The output from 334 enters the complex FFT unit 335, which performs fast Fourier transform for OFDM demodulation. The output from 335 enters the equalizer 336.
The equalizer 336 estimates the transmission line and performs equalization according to the result of estimate. In some cases, information from the synchronizing circuit 333 is also entered to the equalizer 336 to estimate the transmission line. The output from the equalizer 336 enters the demodulator 337, which performs signal point judgment and outputs the estimated value of received bit. The output from 337 enters the deinterleaver 338, which performs deinterleaving to rearrange the string of coded bits according to a prescribed rule. The output from 338 enters the decoder 339, which decodes the error correction code given by the transmitting end.
The output from 339 enters the descrambler 340, which performs descrambling as the inverse conversion of the scrambling performed in the transmitting end. The output from 340 enters the cipher remover 341, which removes cipher made by the transmitting end. The output from 341 enters the CRC checking unit 342, which outputs data from which CRC has been removed and the result of CRC checking of received blocks. The output from 342 enters the received data processing unit 343, which outputs data with the frame structure and slot structure (for transmission through the radio channel) removed, if it judges that there are no errors in the result of CRC checking of received blocks. The output from 343 enters the data input/output processing unit 301, which, after conversion, outputs sound signals in the case of voice communication or data signals in the case of data communication connected to a computer.
In the case where communication control data is contained which has been transmitted from the base station as the called party of radio communication (not shown), the received data processing unit 343 takes out that part, and the output enters the control unit 302 through the receiving system control line 304. The control unit interprets the received control data and controls the action of each unit of the terminal station 300 according to the instruction.
In the case where the ARQ (Automatic Request for Reception) system is employed, the received data processing unit 343 functions as follows. If the input signal from the CRC checking unit 342 contains information that the received block contains no errors, it outputs the received block to the received data processing unit 343 and also outputs to the control unit 302 through the receiving system control line 304, to the effect that the received block contains no errors. Upon receipt of this output, the control unit 302 instructs the transmitting data processing unit 311 through the transmitting system control line 303 to transmit ACK signal to the other OFDM radio equipment (base station) as the called party of radio communication (not shown). The transmitting data processing unit 311 sends ACK signal after performing multiplexing on the transmitting data. The ACK signal is transmitted to the base station by processing of the transmitting system as explained above.
Conversely, if the input signal from the CRC checking unit 342 contains information that the received block contains errors, it does not output the received block to the received data processing unit 343 but outputs to the control unit 302 through the receiving system control line 304, to the effect that the received block contains errors. Upon receipt of this output, the control unit 302 instructs the transmitting data processing unit 311 through the transmitting system control line 303 to transmit NAK signal to the base station as the called party of radio communication (not shown). The transmitting data processing unit 311 sends NAK signal after performing multiplexing on the transmitting data. The NAK signal is transmitted to the base station by processing of the transmitting system as explained above. Upon receipt of this transmission, the base station retransmits the block by which NAK signal has been transmitted.
In the case of stream communication, like voice communication, in which retransmission by the ARQ system is not employed, the received data processing unit 343 functions as follows. If the input signal from the CRC checking unit 342 contains information that the received block contains no errors, it outputs the received block to the received data processing unit 343 as mentioned above. Conversely, if the input signal from the CRC checking unit 342 contains information that the received block contains errors, the received data processing unit 343 discards the received block (handling it as erasure) and performs interpolation by using the received block before one block.
Each part of the transmitting system is connected to the control unit 302 through the transmitting system control line 303, and the control unit 302 controls and monitors various operations for the transmitting system through it (such as on-off of the transmitting system, control and monitor of the RF transmitter 321, fine adjustment of transmitting timing, change of the coding system and signal point mapping, and control of retransmitting). Each part of the receiving system is connected to the control unit 302 through the receiving system control line 304, and the control unit 302 controls and monitors various operations for the receiving system through it (such as on-off of the receiving system, control and monitor of the RF receiver 331, fine adjustment of receiving timing, change of the coding system and signal point mapping, and control of retransmitting).
The conventional OFDM communication system mentioned above works in such a way that the signal to call a terminal station from a base station is transmitted, with all information placed on subcarriers in the transmission band, and the called terminal station receives all the subcarriers to receive the calling signal. This means that the terminal station has to receive and decode the band signal (corresponding to 20 MHz) every 2 ms regardless of presence or absence of data being transmitted and received. It follows, therefore, that large quantities of signals have to be processed even when no information data is transmitted and received. This leads to a waste of batteries in the case where the terminal station is a battery-driven mobile station.
One known way to address this problem is to thin out the frame intervals to be received by negotiation between a base station and a terminal station, instead of receiving control signal frames in all MAC frames.
However, even in the case where the frame intervals to be received are thinned out, the frame period to be received needs reception in the same way as information transmission and reception. Therefore, loads in a terminal station are not so reduced by the above-mentioned way.